This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Ribosomal processing requires a series of endo- and exonucleolytic steps for the production of mature ribosomes, of which most have been described. To ensure ribosome synthesis, 3'end formation of rRNA uses multiple nucleases acting in parallel;however, a similar parallel mechanism had not been described for 5'end maturation. Here, we identify Rrp17p as a previously unidentified 5'-3'exonuclease essential for ribosome biogenesis, functioning with Rat1p in a parallel processing pathway analogous to that of 3'end formation. Rrp17p is required for efficient exonuclease digestion of the mature 5'ends of 5.8S(S) and 25S rRNAs, contains a catalytic domain close to its N terminus, and is highly conserved among higher eukaryotes, being a member of a family of exonucleases. We show that Rrp17p binds late pre-60S ribosomes, accompanying them from the nucleolus to the nuclear periphery, and provide evidence for physical and functional links between late 60S subunit processing and export. A paper describing this work has been published (M. Oeffinger, D. Zenklusen, A. Ferguson, K.E. Wei, A. El Hage, D. Tollervey, B.T. Chait, R.H. Singer, M.P. Rout "Rrp17p Is a Eukaryotic Exonuclease Required for 50 End Processing of Pre-60S Ribosomal RNA" Molecular Cell, 36 (2009) 768-781).